The present invention relates to AST (automatic scan tracking) for a VTR (video tape recorder), and more particularly for such a VTR in which at least two tracks are simultaneously reproduced.
In helical scan VTRs a magnetic tape having recorded tracks is displaced around a drum. Within the drum is a headwheel containing magnetic reproducing heads. The reproducing heads must "track" (stay aligned with) the recorded tracks in order to obtain sufficient amplitude for proper reproduction of the recorded signal. However without some kind of tracking system, tracking errors occur.
A prior art method for controlling tracking errors has the reproducing heads mounted on a bimorph, which in turn is attached to the headwheel, and an oscillating "dither" signal applied to the bimorph to "dither" the heads. If a tracking error occurs, phase and amplitude changes in the reproduced signal are produced, which changes can be detected to provide information as to the direction and magnitude of the error. From this information a tracking control signal is generated and applied to the bimorph to ensure tracking. However, the dither signal may be in the range of the mechanical resonance frequency of the heads and associated mountings, which may cause uncontrolled head vibrations, thereby making controlled tracking difficult. An additional disadvantage of the dither technique is that in the normal tracking mode, the playback head at least partially leaves the recorded track, thereby periodically reducing the carrier signal level and decreasing the carrier-to-noise ratio.
Another AST technique is shown in U.S. Patent Application Ser. No. 416,542, filed Sept. 10, 1982, in the name of K. J. Hamalainen, now U.S. Pat. No. 4,525,750, and assigned to the assignee of the present invention. As disclosed therein, the reproducing heads have a width slightly smaller than the track width. The signals from the reproducing heads are amplitude compared. When mistracking occurs, the amplitudes will differ, thereby providing an error signal to control the bimorph to maintain tracking. This system eliminates head resonance problems; however since the reproducing heads are narrower than the track widths, a reduction in carrier-to-noise ratio still occurs.
It is therefore desirable to provide an AST system that does not cause resonance and maintains the carrier-to-noise ratio.